The present invention relates to methods by which L-tyrosine derived diphenol monomers may be synthesized with significantly improved yield and purity. The present invention also relates to new tyrosine-derived diphenol monomers and polymers prepared therefrom.
Diphenols are monomeric starting materials for polycarbonates, polyiminocarbonates, polyarylates, polyurethanes, and the like. Commonly owned U.S. Pat. No. 5,099,060 discloses amino acid-derived diphenol compounds, useful in the polymerization of polycarbonates and polyiminocarbonates. The resulting polymers are useful as degradable polymers in general, and as tissue compatible bioerodible materials for medical uses, in particular. The suitability of these polymers for this end use application is the result of their polymerization from diphenols derived from the naturally occurring amino acid, L-tyrosine. Commonly owned U.S. Pat. No. 5,216,115 discloses polyarylates polymerized from L-tyrosine derived diphenols that are also useful as tissue compatible bioerodible materials for medical uses.
The L-tyrosine derived diphenol monomers are disclosed by the foregoing patents as being prepared by carbodiimide mediated coupling reactions, as disclosed in Bodanszky, Practice of Peptide Synthesis (Springer-Verlag, New York, 1984) at page 145. The purification of the L-tyrosine derived diphenols has proved to be difficult. The diphenol crystalizes slowly, precipitating as a water-immiscible oil contaminated with side products of the carbodiimide coupling agent. It has since been discovered that the purity and yield of the diphenol could be improved significantly by the addition of an auxiliary nucleophile such as 1-hydroxybenzotriazole hydrate (HOBt).
Although the purity of the crude diphenol improves significantly, the final purification of the compound remains problematic. A major contaminant is the urea by-product of the carbodiimide. For example, the most commonly available, lowest cost carbodiimide, dicyclohexylcarbodiimide (DCC) has proved highly effective in mediating the formation of L-tyrosine derived diphenol compounds, but produces a by-product, dicyclohexylurea (DCU) that is distributed between the aqueous phase and the water-immiscible diphenol containing phase. The complete removal of DCU by extraction and/or precipitation techniques has not been achieved, which is consistent with previous reports indicating that DCU is difficult to remove from a variety of materials. Consequently, column chromatography is required for the complete purification of L-tyrosine derived diphenol compounds prepared by DCC-mediated coupling reactions.
Column chromatography purification techniques are satisfactory for the small scale preparation of L-tyrosine derived diphenol monomers. However, upon scale-up, several disadvantages become evident, including a reduction in yield, the need to use large amounts of organic solvents as eluents and the inherent difficulties of large scale column chromatography. A method is needed by which L-tyrosine derived diphenol compounds may be prepared without difficult to remove by-products. A method by which such by-products may be readily removed would also be desirable.
WO 93/25,571 describes a method by which peptides containing from 2 to 10 amino acids are synthesized by carbodiimide-mediated amino acid coupling reactions. A solvent in which the urea by-product of the carbodiimide is insoluble is utilized as the reaction solvent, so that the urea by-product precipitates and is removed by filtration. However, the process is not applicable to the synthesis of the L-tyrosine-derived diphenols of this application since the urea by-product remains soluble in the reaction solvent to an appreciable extent. Accordingly, there remains a need for an L-tyrosine coupling method in which carbodiimide by-product formation is prevented or, in the alternative, the removal of such by-products is facilitated.
This need is met by the present invention. It has now been discovered that the purity and yield of L-tyrosine derived diphenol compounds prepared by carbodiimide mediated coupling reactions can be improved significantly by utilizing a carbodiimide capable of forming a water-soluble urea by-product in a reaction mixture based on a water-miscible organic solvent. Upon completion of the coupling reaction, combining the reaction mixture with water will precipitate the diphenol product as a water-immiscible organic phase with most of the urea by-product of the carbodiimide remaining in the water-miscible reaction mixture. Any urea by-product remaining in the diphenol phase may be removed by backwashing the organic phase with an aqueous extraction medium.
Therefore, according to one aspect of the present invention, an improved carbodiimide-mediated coupling reaction is provided for the preparation of L-tyrosine derived diphenol compounds. According to this aspect of the invention, a method is provided for preparing diphenol compounds having the structure of Formula I: 
wherein R1 is xe2x80x94CHxe2x95x90CHxe2x80x94 or (xe2x80x94CH2xe2x80x94)n, in which n is zero or an integer from one to eight; and R2 is selected from straight and branched alkyl and alkylaryl groups containing up to 18 carbon atoms; which method includes the step of:
coupling a hydroxyphenyl carboxylic acid having the structure of Formula II: 
with a tyrosine ester having the structure of formula III: 
xe2x80x83in a water-miscible organic reaction solvent containing a carbodiimide capable of forming a water-soluble urea by-product thereby forming a diphenol reaction product. Upon completion of the coupling reaction, the reaction mixture is combined with an amount of water effective to precipitate the diphenol as a water-immiscible organic phase. In this way, two phases are formed, a water-immiscible organic phase containing the bulk of the diphenol reaction product, and an aqueous phase containing the bulk of the water-soluble urea and unreacted starting materials.
In preferred methods in accordance with the present invention, the separated water-immiscible organic phase is washed with an aqueous extraction medium and then separated from the extraction medium. Regardless of whether the washing step is performed, preferred methods in accordance with the present invention precipitate the diphenol reaction product in the form of a water-immiscible phase.
Attendant with the discovery of the improved synthesis method of the present invention, it was also learned that other hydroxyphenyl carboxylic acids could be coupled with L-tyrosine to produce diphenol compounds useful as monomers in the polymerization of tissue compatible bioerodible polymers for biomedical uses. Such hydroxyphenyl carboxylic acids include hydroxycinnamic acid and certain hydroxyphenylalkyl carboxylic acids. The monomers provided by this aspect of the present invention are diphenol compounds having the structure of Formula I wherein R1 is xe2x80x94CHxe2x95x90CHxe2x80x94, or (xe2x80x94CH2xe2x80x94)n, in which n is zero, one, or an integer from three to eight, and R2 is selected from straight and branched alkyl and alkylaryl groups containing up to 18 carbon atoms.
The present invention also includes polycarbonates prepared from the monomers of the present invention by the process disclosed by U.S. Pat. No. 5,198,507 and polyarylates prepared from the monomers of the present invention by the process disclosed by U.S. Pat. No. 5,216,115.
The present invention provides L-tyrosine derived diphenol monomers at significantly reduced cost with sufficient purity for polymerization to high molecular weight tissue compatible bioerodible polymers for biomedical use.